Zooming in on black holes is the main
mission for the newly installed instrument GRAVITY at ESO’s Very Large
Telescope in Chile. During its first observations, GRAVITY successfully
combined starlight using all four Auxiliary Telescopes. The large team
of European astronomers and engineers, led by the Max Planck Institute
for Extraterrestrial Physics in Garching, who designed and built
GRAVITY, are thrilled with the performance. During these initial tests,
the instrument has already achieved a number of notable firsts. This is
the most powerful VLT Interferometer instrument yet installed.

The GRAVITY
instrument combines the light from multiple telescopes to form a
virtual telescope up to 200 metres across, using a technique called interferometry. This enables the astronomers to detect much finer detail in astronomical objects than is possible with a single telescope.

Since the summer of 2015, an international team of astronomers and engineers led by Frank Eisenhauer (MPE,
Garching, Germany) has been installing the instrument in specially
adapted tunnels under the Very Large Telescope at ESO’s Paranal
Observatory in northern Chile [1].
This is the first stage of commissioning GRAVITY within the Very Large
Telescope Interferometer (VLTI). A crucial milestone has now been
reached: for the first time, the instrument successfully combined
starlight from the four VLT Auxiliary Telescopes [2].

“During its first light, and for the first time in the history of
long baseline interferometry in optical astronomy, GRAVITY could make
exposures of several minutes, more than a hundred times longer than
previously possible,” commented Frank Eisenhauer. “GRAVITY will
open optical interferometry to observations of much fainter objects,
and push the sensitivity and accuracy of high angular resolution
astronomy to new limits, far beyond what is currently possible.”

As part of the first observations the team looked closely at the bright, young stars known as the Trapezium Cluster,
located in the heart of the Orion star-forming region. Already, from
these first commissioning data, GRAVITY made a small discovery: one of
the components of the cluster was found to be a double star [3].

The key to this success was to stabilise the virtual telescope for
long enough, using the light of a reference star, so that a deep
exposure on a second, much fainter object becomes feasible. Furthermore,
the astronomers also succeeded in stabilising the light from four
telescopes simultaneously — a feat not achieved before.

GRAVITY can measure the positions of astronomical objects on the
finest scales and can also perform interferometric imaging and
spectroscopy [4].
If there were buildings on the moon, GRAVITY would be able to spot
them. Such extremely high resolution imaging has many applications, but
the main focus in the future will be studying the environments around
black holes.

In particular, GRAVITY will probe what happens in the extremely
strong gravitational field close to the event horizon of the
supermassive black hole at the centre of the Milky Way — which explains
the choice of the name of the instrument. This is a region where
behaviour is dominated by Einstein's theory of general relativity.
In addition, it will uncover the details of mass accretion and jets —
processes that occur both around newborn stars (young stellar objects)
and in the regions around the supermassive black holes at the centres of
other galaxies. It will also excel at probing the motions of binary
stars, exoplanets and young stellar discs, and in imaging the surfaces
of stars.

So far, GRAVITY has been tested with the four 1.8-metre Auxiliary
Telescopes. The first observations using GRAVITY with the four 8-metre
VLT Unit Telescopes are planned for later in 2016.

The GRAVITY consortium is led by the Max Planck Institute for
Extraterrestrial Physics, in Garching, Germany. The other partner
institutes are:

[1] The VLTI tunnels and beam-combining room have recently undergone
significant construction work to accommodate GRAVITY as well as to
prepare for other future instruments.

[2] It would be more accurate to call
this step “first fringes” as the milestone was the first successful
combination of light from the different telescopes so that the beams
interfered and fringes were formed and recorded.

[3] The newly discovered double star is Theta1 Orionis F, and the observations were made using the nearby brighter star Theta1 Orionis C as the reference.

[4] GRAVITY aims to measure the
positions of objects on scales of order ten microarcseconds, and perform
imaging with four milliarcsecond resolution.